Synthetic gels (e.g., synthetic hydrogels) constitute a class of materials useful in biomedicine. One of the first such applications for hydrogels was soft contact lenses invented by Wichterle and Lim. The gel used by Wichterle and Lim was composed of a covalently crosslinked poly(hydroxyethyl methacrylate (p(HEMA)). Such covalently-linked gels typically are typically quite robust but suffer from inability to self-heal when damaged. They also cannot be injected because they don't flow at relevant timescales, even under a high shear stress.
To complement these materials, metal-ligand coordination-linked gels have been developed where coordination complexation between metal ions and ligands were one or the sole crosslinking motif (Holten-Andersen et al., Proceedings of the National Academy of Sciences of the United States of America, 2011, 108, 2651-2655; Holten-Andersen et al., Journal of Materials Chemistry B, 2014, 2, 2467-2472; Barrett et al., Advanced functional materials, 2013, 23, 1111-1119; Fullenkamp et al., Macromolecules, 2013, 46, 1167-1174; Menyo et al., Soft matter, 2013, 9:10314-10323). In particular, much of the work thus far has focused primarily on demonstrating the mechanical properties of gels using different metal-ligand pairs. For example, Fullenkamp et al. studied histidine hydrogels with Zn2+, Cu2+, Co2+, and Ni2+ as the central atoms in coordination complexes (Fullenkamp et al., Macromolecules, 2013, 46, 1167-1174); Holten-Andersen et al. studied 3,4-dihydroxyphenylalanine (DOPA) hydrogels with Fe3+, V3+, and Al3+ the central atoms (Holten-Andersen et al., Journal of Materials Chemistry B, 2014, 2, 2467-2472); and Menyo et al. studied hydrogels using DOPA and chemical modifications of DOPA using Fe3+ as metal centers (Menyo et al., Soft matter, 2013, 9:10314-10323). While these metal-ligand coordination-linked gels are able to flow at a high shear stress and to self-heal due to the dynamic nature of the coordination bonding, they typically lack robustness and storage modulus of covalently linked gels. Therefore, there remains a need for new gels with improved properties.